A number of diseases have been identified to date in which natural resistance to infection exists in the human population. Alter and Moyer, J. Acquir. Immune Defic. Syndr. Hum Retrovirol. 18 Suppl. 1:S6-10 (1998) report hepatitis C viral infection (HCV) rates as high as 90% in high-risk groups such as injecting drug users. However, the mechanism by which the remaining 10% are apparently resistant to infection has not been identified in the literature. Proteins that play a role in HCV infection include the 2-prime, 5-prime oligoadenylate synthetases. OASs are interferon-induced proteins characterized by their capacity to catalyze the synthesis of 2-prime, 5-prime oligomers of adenosine (2-5 As). Hovanessian et al., EMBO 6: 1273-1280 (1987) found that interferon-treated human cells contain several OASs corresponding to proteins of 40 (OAS1), 46 (OAS1), 69, and 100 kD. Marie et al., Biochem. Biophys. Res. Commun. 160:580-587 (1989) generated highly specific polyclonal antibodies against p69, the 69-kD OAS. By screening an interferon-treated human cell expression library with the anti-p69 antibodies, Marie and Hovanessian, J. Biol. Chem. 267: 9933-9939 (1992) isolated a partial OAS2 cDNA. They screened additional libraries with the partial cDNA and recovered cDNAs encoding two OAS2 isoforms. The smaller isoform is encoded by two mRNAs that differ in the length of the 3-prime untranslated region.
Northern blot analysis revealed that OAS2 is expressed as four interferon-induced mRNAs in human cells. The predicted OAS2 proteins have a common 683-amino acid sequence and different 3-prime termini. According to SDS-PAGE of in vitro transcription/translation products, two isoforms have molecular masses of 69 and 71 kD. Both isoforms exhibited OAS activity in vitro. Sequence analysis indicated that OAS2 contains two OAS1-homologous domains separated by a proline-rich putative linker region. The N- and C-terminal domains are 41% and 53% identical to OAS1, respectively.
By fluorescence in situ hybridization and by inclusion within mapped clones, Hovanian et al., Genomics 52: 267-277 (1998) determined that the OAS1, OAS2, and OAS3 genes are clustered with a 130-kb region on 12q24.2. 2-5 As bind to and activate RNase I, which degrades viral and cellular RNAs, leading to inhibition of cellular protein synthesis and impairment of viral replication.
A fourth human OAS gene, referred to as OASL, differs from OAS1, OAS2 and OAS3 in that OASL lacks enzyme activity. The OASL gene encodes a two-domain protein composed of an OAS unit fused to a 164 amino acid C-terminal domain that is homologous to a tandem repeat of ubiquitin. (Eskildsen et al., Nuc. Acids Res. 31:3166-3173, 2003; Kakuta et al., J. Interferon & Cytokine Res. 22:981-993, 2002.)
Because of their role in inhibiting viral replication and viral infection, there is a need in the art for methods and compositions that suppress viral replication related to OAS1 activity, including a profound need for inhibitor-based therapies that suppress HCV replication.